Insects That Get Inside of Black Walnut Hulls

A variety of insects attack the shoots and foliage of black walnuts (Juglans nigra), but a limited number feed on the nuts. Their larvae bore into the hulls, eating the interior or causing the nuts to drop prematurely. There are pesticides registered for controlling them, although there may be restrictions where you live. You can grow black walnuts in U.S. Department of Agriculture plant hardiness zones 4 through 9.

Walnut Curculio

The walnut curculio (Conotrachelus retentus) is the most common insect that eats its way into black walnut hulls. This reddish brown insect, about 1/5 inch long, has two small white spots on its wing covers and a long snout. Its dirty white larvae are legless. Adult female curculio lay their eggs on young nuts in late spring and early summer. Their larvae bore into the developing nuts, making some drop and causing smaller kernels in those that remain on the tree. They leave a small hole on the side of the nut hull.

Walnut Husk Fly

The walnut husk fly (Rhagoletis completa) is the second insect likely to eat its way into black walnut husks. Their maggots spend winters in the ground and emerge as flies in mid to late summer. Adult husk flies lay eggs in the husks of black and Persian walnuts (Juglans regia), hardy in USDA zones 5b through 6b. The legless maggots feed on the hulls, staining the shells. The light brown flies, smaller than a housefly, have dark bars crossed on their two transparent wings. Their legless, pale yellow larvae grow up to 1/2 inch long.

Pecan Nut Casebearer

While the pecan nut casebearer (Acrobasis nuxvorella) typically infests pecans (Carya illinoensis) hardy in USDA zones 5b through 9a, hence its name, it can also infect black walnuts. A small, brownish-gray moth about 1/3 inch long, it has a tuft or ridge of dark scales across the middle of its front wings. It spends the winter in a cocoon at the base of nut buds and emerges in spring to lay eggs on developing nuts. Its larvae enter the stem end of the hulls and eat the interior.

Control

To control walnut curculio, spray your tree with a pesticide containing the active ingredient esfenvalerate, lambda-cyhalothrin, carbaryl or phosmet. Do this just after your tree yields slim, cylindrical clusters of flowers, signifying that is ready to receive pollen. Home growers typically ignore the walnut husk fly because they don’t affect the nutmeats. If you do wish to use a pesticide, University of California entomologists recommend one containing the active ingredient spinosad. For the pecan nut casebearer, Texas A&M; entomologists recommend the organic Bacillus thuringiensis or a chemical pesticide containing the active ingredient carbaryl, malathion or spinosad. State registration for chemical pesticides change from time to time as do application rates and how they should be applied. Consult your local agriculture extension agent before using them.